1. Field of the Invention
The present invention relates generally to battery protection circuits, and more specifically, to a charge-pump biased battery protection circuit having reduced leakage, improved efficiency and reduced complexity.
2. Background of the Invention
Devices incorporating rechargeable batteries, such as Lithium ion (Li+) batteries and Nickel metal hydride (NiMH) batteries, typically incorporate circuits that protect the battery during charging and discharging. U.S. Pat. No. 6,670,790 to Stellberger, incorporated herein by reference, details several such battery protection circuits, which typically include one or more pass transistors with gate control electronics that limit the current during discharging and charging of the battery. The protection circuits also typically completely disables the discharging action when the battery output voltage falls to too low a level, i.e., when the internal resistance of the battery rises too high due to the level of discharge.
However, such battery protection circuits are inefficient in that the voltage drop across the pass transistor is substantial, causing a waste of available battery energy as dissipation in the pass transistor during discharge. In particular, during intervals of high current drain, the energy loss and power dissipation due to the pass transistor voltage drop are typically substantial. The result is reduced battery cycle life as well as a requirement for pass transistors and/or heat management with the capability to handle the dissipation during such intervals, making it difficult to integrate such circuits on a die with other circuits.
In the improvement introduced in the above-incorporated U.S. Patent Application, the number of required pass transistors is reduced to one, by control of the bulk (body) voltage of a single enhancement field-effect transistor (FET) so that conduction can be provided in either direction through the pass transistor. However, the management of the bulk voltage requires two switches as shown in FIG. 2 of the above-incorporated U.S. patent application and the separate control of the switches introduces further complexity in the control circuit. Further, the voltage drop across even a single FET can still have a significant impact on efficiency and power dissipation during intervals of heavy discharge.
Therefore, it would be desirable to provide a battery protection circuit that has improved efficiency and reduced power dissipation. It would further be desirable to provide such a battery protection circuit that uses a single pass transistor, but has reduced control circuit complexity.